1. Field of the Invention
The present invention relates to modulation of m-bit information words into a signal and demodulation of the modulated signal, and more particularly, to a method of generating and allocating a (1, 8, 8, 12) code having effective DC suppression in a code word stream, a modulation and demodulation method, a demodulation apparatus therefor, in a run length limited (RLL) code to be used in optical disc recording and/or reproducing apparatuses which require particularly high density recording and/or reproducing.
2. Description of the Related Art
In optical recording and/or reproducing apparatuses, a run length limited (RLL) code represented by (d, k, m, n) is widely used for modulating original information into a signal appropriate for an optical disc and demodulating the signal reproduced from the optical disc into the original information. Types of optical discs currently in use include a compact disc (CD), and a digital versatile disc (DVD), and, at present, a high density disc (HD-DVD) which will have a higher density recording than that of the DVD is under development.
Among RLL codes which can be used in the HD-DVD, the characteristic of a (1, 7, 2, 3) code is the fact that its run length is restricted since the minimum run length is 2T(d=1), and the maximum run length is 8T(k=7). Its code rate R is R=m/n=2/3. T is an interval between each bit in a code word.
As shown in FIG. 1A, 2 bits of data are converted into 3 channel bits. When the run length is violated , that is, when user data "00" is followed by user data "00" or "01", or when user data "10" is followed by user data "00" or "01", the channel bits are replaced by using the conversion table of FIG. 1B.
Since a code word by itself cannot suppress the DC component of the code word in the conventional (1, 7, 2, 3) code, the variation of the Digital Sum Value (DSV) does not converge with DC value "0", as shown in FIG. 2 which illustrates the DSV variation curve of the (1, 7, 2, 3) code, when synchronization signals are not added. When this code is used in an optical disc system, a data slicer (also referred to as a data digitizing circuit) which converts a radio frequency signal picked up from the disc into a binary signal cannot properly operate. In addition, the lower frequency component of the code signal flows in a servo unit, which includes a tracking control unit and a focusing control unit, to create a bad influence, and lowers the reliability of the system.